Auto merge of rust-lang#121001 - nyurik:optimize-core-fmt, r=<try>

perf: improve write_fmt to handle simple strings

Per `@dtolnay` suggestion in serde-rs/serde#2697 (comment) - attempt to speed up performance in the cases of a simple string format without arguments:

```rust
write!(f, "text")  ->  f.write_str("text")
```

```diff
+ #[inline]
  pub fn write_fmt(&mut self, f: fmt::Arguments) -> fmt::Result {
+     if let Some(s) = f.as_str() {
+         self.buf.write_str(s)
+     } else {
          write(self.buf, f)
+     }
  }
```

* Hopefully it will improve the simple case for the rust-lang#99012
* Another related (original?) issues rust-lang#10761
* Previous similar attempt to fix it by by `@Kobzol` rust-lang#100700

CC: `@m-ou-se` as probably the biggest expert in everything `format!`

Author: bors

library/core/src/fmt/mod.rs

@@ -201,14 +201,22 @@ pub trait Write {
         impl<W: Write + ?Sized> SpecWriteFmt for &mut W {
             #[inline]
             default fn spec_write_fmt(mut self, args: Arguments<'_>) -> Result {
-                write(&mut self, args)
+                if let Some(s) = args.as_const_str() {
+                    self.write_str(s)
+                } else {
+                    write(&mut self, args)
+                }
             }
         }
 
         impl<W: Write> SpecWriteFmt for &mut W {
             #[inline]
             fn spec_write_fmt(self, args: Arguments<'_>) -> Result {
-                write(self, args)
+                if let Some(s) = args.as_const_str() {
+                    self.write_str(s)
+                } else {
+                    write(self, args)
+                }
             }
         }
 
@@ -430,6 +438,15 @@ impl<'a> Arguments<'a> {
             _ => None,
         }
     }
+
+    /// Same as [`Arguments::as_str`], but will only return `Some(s)` if it can be determined at compile time.
+    #[must_use]
+    #[inline]
+    const fn as_const_str(&self) -> Option<&'static str> {
+        let s = self.as_str();
+        // SAFETY: both cases are valid as the result
+        if unsafe { core::intrinsics::is_val_statically_known(s.is_some()) } { s } else { None }
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -1582,8 +1599,9 @@ impl<'a> Formatter<'a> {
     /// assert_eq!(format!("{:0>8}", Foo(2)), "Foo 2");
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[inline]
     pub fn write_fmt(&mut self, fmt: Arguments<'_>) -> Result {
-        write(self.buf, fmt)
+        if let Some(s) = fmt.as_const_str() { self.buf.write_str(s) } else { write(self.buf, fmt) }
     }
 
     /// Flags for formatting
@@ -2272,8 +2290,13 @@ impl Write for Formatter<'_> {
         self.buf.write_char(c)
     }
 
+    #[inline]
     fn write_fmt(&mut self, args: Arguments<'_>) -> Result {
-        write(self.buf, args)
+        if let Some(s) = args.as_const_str() {
+            self.buf.write_str(s)
+        } else {
+            write(self.buf, args)
+        }
     }
 }
 

library/core/src/lib.rs

@@ -176,6 +176,7 @@
 #![feature(ip)]
 #![feature(ip_bits)]
 #![feature(is_ascii_octdigit)]
+#![feature(is_val_statically_known)]
 #![feature(isqrt)]
 #![feature(maybe_uninit_uninit_array)]
 #![feature(non_null_convenience)]